This invention relates to an apparatus and method of measuring both distortion and wavefront-aberration in optical systems such as a ringfield projection lithography camera. More particularly, the invention employs a point diffraction interferometer that can simultaneously make both measurements without reconfiguration.
Extreme ultraviolet lithography is a promising technology for integrated circuit fabrication for feature sizes less than 0.1 xcexcm. It is an optical projection lithography scheme using short wavelength radiation with all-reflective optics based on multilayer coatings.
Optical metrology is the characterization of systems, surfaces, and/or materials using optical methods. An area of optical metrology relates to the use of an interferometer to measure the quality of a test optic, such as a single or multiple element mirror or lens system.
One important recent application of optical metrology is the testing and alignment of projection optics for photolithography systems. Modern photolithography systems used to fabricate integrated circuits must continually image smaller features. In pursuit of this goal, systems are confronted with the diffraction limit determined in part by the wavelength of the light employed. To meet the challenge of imaging ever smaller features, photolithographic systems must employ successively shorter wavelengths. Over the history of integrated circuit fabrication technology, photolithography systems have moved from visible to ultraviolet and may eventually move to even shorter wavelengths such as extreme ultraviolet or to yet shorter X-ray radiation.
As with all optical imaging systems, photolithographic optics may have various aberrations such as spherical astigmatism and coma present. These aberrations must be identified and removed during the fabrication and/or alignment of the projection optics, or the projection optics would introduce substantial blurring in the image projected onto the wafer.
Interferometers may be employed to test the projection optics for various aberrations. Conventional interferometers, based upon the Michelson design for example, employ a single coherent light source (at an object plane) which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram, and the resultant wavefront with, for example, Zernike polynomials, indicates the presence of aberrations.
The reference wave of the interferometer should be xe2x80x9cperfectxe2x80x9d; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optical elements through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always contains some artifacts from the optical elements through which the reference wave passes. While these artifacts, in theory, can be separated from the interferogram, it is usually impossible to know that a subtraction produces a truly accurate interferogram.
To address this problem, the phase-shifting point diffraction interferometer has been developed; it is a variation of the conventional point diffraction interferometer in which a transmission grating has been added to greatly improve the optical throughput of the system and add phase-shifting capability. The phase-shifting point diffraction interferometer (PS/PDI) is described in H. Medecki, et al., xe2x80x9cPhase-Shifting Point Diffraction Interferometerxe2x80x9d, Optics Letters, 21(19), 1526-28 (1996), E. Tejnil, et al., xe2x80x9cAt-Wavelength Interferometry for EUV Lithography,xe2x80x9d J. Vacuum Science and Tech. B, 15, 2455-2461 (1997), K. A. Goldberg, et al., xe2x80x9cCharacterization of an EUV Schwarzchild Objective Using Phase-Shifting Point Diffraction Interferometry,xe2x80x9d Proceeding SPIE, 3048, 264-270 (1997), E. Tejnil, et al., xe2x80x9cPhase-Shifting Point Diffraction Interferometry for At-Wavelength Testing of Lithographic Optics,xe2x80x9d OSA Trends in Optics and Photonics: Extreme Ultraviolet Lithography, Optical Society of America, Washington, D.C., 4, 118-123 (1996), K. A. Goldberg, xe2x80x9cExtreme Ultraviolet Interferometry,xe2x80x9d doctoral dissertation, Dept. of Physics, Univ. of California, Berkeley (1997), and in the U.S. Pat. No. 5,835,217 xe2x80x9cPhase-Shifting Point Diffraction Interferometer,xe2x80x9d Hector Medecki, which are all incorporated herein by reference.
As with any multi-element diffraction limited imaging system, alignment is a crucial aspect in the development of extreme ultraviolet (EUV) projection lithography systems. For commercial quality performance, the alignment must also address the problem of distortion. Distortion in a projection optical system is related to image placement errors that vary as a function of position in the image field. For example, a uniformly distributed grid of points or object as shown in FIG. 6A would be imaged as a non-uniform grid in a system with distortion. Two common forms of distortion are the xe2x80x9cbarrelxe2x80x9d and xe2x80x9cpincushionxe2x80x9d configurations which are depicted (in exaggerated form) in FIGS. 6B and 6C, respectively.
The conventional method of measuring distortion in photolithographic lenses involves printing wafers whereas the conventional method of measuring wavefront-aberration is interferometry as indicated above. It is known in the art, however, that performing the alignment based solely on wavefront-aberration minimization can introduce a significant amount of distortion into the system. This distortion could be several orders of magnitude larger than that specified by the optical design. This distortion could be too large to correct based on subsequent print measurement without affecting the wavefront performance, which would require further wavefront interferometry to correct. As is apparent, it would be advantageous to be able to measure both wavefront-aberration and distortion using a single instrument. Implementing the conventional printing method for measuring distortion on an EUV interferometry beamline, however, is not feasible due to the severely disparate illumination required for interferometry and full-field printing. When using point diffraction interferometry to characterize an example lithographic optical system, a micron sized illumination spot is required in the object plane, whereas for full-field printing 4-inch-wide arc-field illumination might be required.
The present invention is based in part on the recognition that the phase-shifting point diffraction interferometer can also be employed to directly measure distortion. Indeed, the improved PS/PDI of the present invention can measure wavefront aberration and distortion in a single instrument without any reconfiguration. This greatly simplifies the complicated task of aligning diffraction limited optical systems.
In one embodiment, the invention is directed to system for interferometric distortion measurements that defines an optical path, said system including:
(a) a test optic with an extended field of view;
(b) a source of electromagnetic radiation in the optical path;
(c) an object-plane pinhole array comprising a plurality of object pinholes with known positions located between the test optic region and the source of electromagnetic radiation whereby energy passing through any one of the plurality of object pinholes is spatially coherent;
(d) a beam divider in the optical path for dividing electromagnetic radiation from the source into a reference beam and a test beam;
(e) an image-plane mask array that is positioned in the image plane of the test optic wherein the image-plane mask array comprises a plurality of test windows and corresponding reference pinholes of known positions, wherein the positions of the plurality of object pinholes in the object-plane pinhole array register with those of the plurality of test windows in image-plane mask array to account for optic demagnification; and
(f) means for directing electromagnetic radiation from the source of electromagnetic radiation into a first object pinhole of the object-plane pinhole array to thereby create a first corresponding test-beam image on the image-plane mask array.
The system can also include means for directing the electromagnetic radiation successively through the first pinhole and thereafter to one or more other pinholes of the plurality of pinholes of the object-plane mask array.
In another embodiment, the invention is directed to a method of measuring the distortion of a test optic defining multiple field points which includes the steps of:
(a) providing a point diffraction interferometer defining an optical path that includes (i) a test optic having a test optic region at which the test optic defines multiple field points, (ii) a source of electromagnetic radiation in the optical path, (iii) an object-plane pinhole array comprising a plurality of object pinholes of known positions located between the test optic region and the source of electromagnetic radiation whereby energy passing through any one of the plurality of object pinholes is spatially coherent, (iv) a beam divider in the optical path for dividing electromagnetic radiation from the source into a reference beam and a test beam, (v) an image-plane mask array that is positioned in the image plane of the test optic region wherein the image-plane mask array comprises a plurality of test windows and corresponding reference pinholes of known positions wherein the positions of the plurality of object pinholes in the object-plane pinhole array register with those of the plurality of test windows in the image-plane mask array to account for optic demagnification;
(b) directing electromagnetic radiation from the source of electromagnetic radiation into a first object pinhole of the plurality of object pinholes of the object-plane pinhole array to thereby create a corresponding first test-beam image and reference-beam image on the image-plane mask array;
(c) measuring the separation distance between and the orientation of the test-beam image and the center of the test-beam window; and
(d) repeating steps (b) and (c) for at least one other object pinhole of the plurality of object pinholes of the object-plane pinhole array.
In a further embodiment the invention is directed to a method of measuring the distortion of a test optic which includes the steps of:
(a) providing a diffraction interferometer defining an optical path that includes (i) an imaging system with real conjugates, (ii) a test optic with an extended field of view, (iii) a source of electromagnetic radiation in the optical path, and (iv) a beam divider in the optical path for dividing the electromagnetic radiation into at least two beams;
(b) measuring a first interferometric pattern of the two beams at a first arbitrary field point within the optic""s field of view, said point providing a reference wherein the distortion is defined to be zero;
(c) measuring a second interferometric pattern of the two beams at a second arbitrary field point within the optic""s field of view; and
(d) calculating the distortion in the test optic by comparing the measurements made in step (c) to the measurement made in step (b).
In yet another embodiment, the invention is directed to a method of measuring the distortion of a test optic which includes the steps of:
(a) providing a diffraction interferometer defining an optical path that includes (i) means for separating a beam of light into a reference beam and a test beam and (ii) a test optic with an extended field of view;
(b) measuring a first interferometric pattern of the two interferometric beams at a first arbitrary field point within the optic""s field of view, said point providing a reference wherein the distortion is defined to be zero;
(c) measuring a second interferometric pattern of the two beams at a second arbitrary field point within the optic""s field of view; and
(d) calculating the distortion in the test optic by comparing the measurements made in step (c) to the measurement made in step (b).